A particular problem experienced by DSL connections operating at increasingly higher frequencies is known as FEXT (Far End CROSS Talk) (note that the VDSL2 standard uses higher frequencies than say ADSL1, and G.fast uses higher frequencies than VDSL2, etc.—in particular VDSL2 uses an additional band of frequencies for upstream transmissions at much higher frequencies than those used for upstream transmissions in ADSL).
In a typical arrangement within an access network, a single Access Node (AN) (e.g. a Digital Subscriber Line Access Multiplexor (DSLAM) located in a street cabinet) is connected via multiple respective copper pairs to multiple respective Customer Premises Equipment (CPE) DSL modems, and the multiple copper pairs typically extend in close proximity to one another for at least a part of their extent (e.g. within a common cable binder). Each of the connecting lines can be considered as having a remote end at which the CPE device is connected and a near (or access network) end at which the AN is connected.
In such an arrangement (as will be well understood by persons skilled in the art of telecommunications), FEXT is noise (induced into a first line from an adjacent line) as measured by a device at a first end (remote/near) of the first line which is caused by signals transmitted by a modem at the far end (near/remote) of the adjacent line. Thus in a case where an AN serves (via an AN VDSL2 modem) a first CPE (having a CPE VDSL2 modem) via a first copper (or metallic) pair using VDSL2 and serves (via an AN G.fast modem) a second CPE (having a CPE G.fast modem) via a second copper (or metallic) pair using G.fast, the G.fast modem can cause FEXT into the CPE VDSL2 modem at the remote end of the first copper pair by virtue of any transmissions having sufficient spectral power at frequencies of use to the VDSL2 modem, and similarly the AN VDSL2 modem can cause FEXT into the CPE G.fast modem at the remote end of the first copper pair. Moreover, the CPE G.fast modem at the remote end of the second copper pair, can cause FEXT into the AN VDSL2 modem at the near end of the first copper pair, and similarly the CPE VDSL2 modem at the remote end of the first copper pair can cause FEXT into the AN G.fast modem at the near end of the second copper pair.
In contrast to FEXT, the other principal type of cross talk affecting communications over copper access networks is known as NEXT (Near End cross(X)Talk). NEXT is similar to FEXT except that it arises primarily from transmissions from a modem in an Access Node (AN) being received as noise (via electromagnetic induction from one line to another) by another modem in the AN (note NEXT can also arise from transmissions from a modem in different AN to that in which the “victim” modem is located, but normally this only occurs where the AN's are geographically very close to one another—e.g. collocated within the same housing structure such as a street cabinet or an Exchange building or Central Office (CO)), or by transmissions from a CPE modem being received as noise by another neighboring CPE modem (where neighboring here means that the lines terminated by the respective CPE modems are sufficiently close together for at least part of their extent that signals can “leak” from one line to the other to a significant degree).
In most modern access networks operating DSL, NEXT is not a big problem because all of the protocols employed contemporaneously within the network (e.g. SHDSL, ADSL, VDSL etc.) use an aligned Frequency Division Duplex (FDD) plan whereby if a frequency range set aside for upstream transmissions in one aligned protocol (e.g. ADSL) is used by another protocol, it will only be used by that other protocol for upstream transmissions, and similarly if a frequency range set aside in one protocol for downstream transmissions is used by another protocol, it will only be used by that other protocol for downstream transmissions. As such, no modems within an Access Node (in say the United Kingdom—if they are operating in accordance with the UK's Access Network Frequency Plan (ANFP)) will be transmitting signals in “upstream” designated frequency bands and similarly no CPE modems will be transmitting in “downstream” designated bands (or in other words, for all frequencies at which an AN modem is transmitting (downstream), no AN modems attempt to receive (upstream) transmissions from CPE modems, and for all frequencies at which a CPE modem is transmitting (upstream), no CPE modems attempt to receive (downstream) transmissions from an AN modem).
However, in the proposed G.fast protocol (e.g. as specified in ITU standards G.9700 and G.9701), rather than using an FDD protocol, a TDD protocol is used instead. This means that when an AN modem is transmitting (unless it is otherwise constrained by a PSD mask) it will transmit in all frequencies including those assigned to upstream transmissions in VDSL2, and similarly when a CPE modem is transmitting it will transmit in all frequencies including those designated as downstream bands. Accordingly, when an AN modem is transmitting, it may cause NEXT into neighboring VDSL AN modems in overlapping VDSL “upstream” designated frequency bands, and similarly when a CPE modem is transmitting it may cause NEXT into neighboring VDSL CPE modems in overlapping VDSL “downstream” designated frequency bands. Moreover, in many circumstances, the effect of NEXT will be more disruptive to neighboring lines than FEXT would have been for the same level of power spectral density (PSD)—i.e. in many circumstances, the NEXT coupling from a near end to near end device (i.e. a remote CPE device to another remote CPE device, or an AN modem to another AN modem) is stronger than the FEXT coupling from the corresponding far end device (i.e. an AN modem to a CPE device of a neighboring line, or a CPE device to an AN modem of a neighboring line).
To account for this possibility for undesirable NEXT to occur, it has been proposed that any line operating using G.fast should basically use only frequencies above the maximum usable frequency for VDSL2 if there are any neighboring lines operating using VDSL2 (on the other hand if there are no neighboring VDSL2 lines, then the G.fast line can use all available frequencies usable according to the G.fast standard—including those below the maximum usable frequency for VDSL2). This may be implemented by providing a suitable PSD mask which constrains the modems from operating above a certain specified PSD level at specified frequencies (namely those at which a neighboring VDSL2 modem could operate). This should ensure that there will be no undesirable crosstalk (neither FEXT nor NEXT) from a G.fast line (whether from the AN G.fast modem or from the CPE G.fast modem) to a neighboring VDSL2 line/modem. However, this is a very conservative approach and results in G.fast lines operating at data rates significantly lower than they otherwise could have done if this constraint were not placed upon them.